The present invention pertains generally to impact resistant styrenic resin compositions and to processes for the preparation thereof. More particularly, such invention specifically relates to rubber-modified styrenic polymers in which the dispersed grafted rubbery polymer particles exhibit at least two distinctly different rubber particle morphologies and satisfy certain critical size and content requirements for each of the respective types of rubber particles.
Rubber modified polystyrene resins constitute a generally known category of materials which find practical utility in a wide variety of injection molding and extrusion applications. In recent times, substantial attention has been directed to improving the overall performance properties and characteristics of such resin materials by modifying and controlling the average size, the size distribution and the morphology of the grafted rubber particles that are dispersed throughout the continuous polystyrene matrix phase of such resin compositions.
For example, in Dupre's U.S. Pat. No. 4,146,589 (issued Mar. 27, 1979), there is disclosed a continuous mass polymerization process for preparing a bimodal particle sized high impact polystyrene (HIPS) composition in which a first diene rubber/styrene monomer solution is partially polymerized to form 0.5 to 1 micron-sized dispersed rubber particles; a second diene rubber/styrene monomer solution is partially polymerized to form 2 to 3 micron-sized dispersed rubber particles; and the two resulting partially polymerized mixtures are then combined and further polymerized to the desired final degree of styrene monomer to polymer conversion. According to this particular reference, a good gloss/toughness balance occurs with the resulting resins when the 0.5 to 1 micron particles constitute from 70 to 95 percent of the rubber particles and the remaining 5 to 30 percent is composed of the 2 to 3 micron particles.
Echte et al.'s U.S. Pat. No. 4,493,922 (issued Jan. 15, 1985) is also concerned with bimodal rubber particle-sized HIPS compositions. In particular, the compositions of concern therein are predominantly (i.e., 60 to 95 weight percent on a butadiene weight basis) composed of 0.2 to 0.6 micron rubber particles having a so-called "capsule" morphology (also variously referred to in the art as "single occlusion" or "core/shell" morphology) in combination with from 5 to 40 weight percent (on a butadiene only weight basis) of 2 to 8 micron particles having a cellular or "coil" particle morphology.
Kasahara et al.'s U.S. Pat. No. 5,039,714 is quite similar to the above-discussed Echte et al. patent insofar as it is premised upon a bimodal HIPS composition containing a relatively small (i.e., 0.1 to 0.6 micron) particle size dispersed rubber component having a single occlusion (or "capsule") morphology and a relatively larger dispersed rubber component having a cellular morphology which Kasahara et al. also refers to as a "salami" morphology. Kasahara et al. does differ from Echte et al. in that the volume averaged particle size of its cellular particle component is required to be from 0.7 to 1.9 micron.
Other known types of dispersed rubber particle morphology which can be obtained under the proper graft polymerization conditions in the preparation of rubber modified polystyrene resins include the so-called entanglement (or "labyrinth") morphology (i.e., wherein polystyrene inclusions within the rubber particle appear as irregularly curved, elongated domains of polystyrene separated by relatively thinner elongated layers of rubber) and the so-called concentric shell or "onion skin" morphology which is characterized by alternating generally spherical or elliptical polystyrene layers and rubber layers.
HIPS resins having these latter types of rubber particle morphology are described in some detail in published European Application Number 0048390 (Mar. 31, 1982) and are taught as being particularly effective for use in blends with polyphenylene ether resins. According to this reference, the overall average particle size of the dispersed rubber particles should be in the range of from 0.5 to 3 micron.
More recently (i.e., in published European Application Number 0460541 of Dec. 11, 1991), entanglement rubber particle morphology has been taught as being an equivalent alternative to cellular rubber particle morphology in the context of certain trimodal particle-sized HIPS compositions. In accordance with this latter reference, an improved property balance as between gloss and impact strength is provided by HIPS compositions which contain from 40 to 98 weight percent of 100 to 600 nm (0.1 to 0.6 micron) capsule morphology rubber particles in combination with from 2 to 60 weight percent of a mixture of cellular and/or entanglement morphology rubber particles, from 1 to 60 weight percent of which have a size of from 200 to 1,200 nm (0.2 to 1.2 micron) and 40 to 99 weight percent of which have a size of from 1,200 to 8,000 nm (1.2 to 8 micron).